One tradition of the Japanese culture is origami, or paper folding. There is a special origami which creates the lover´s knot. This origami seals the message tightly within, and by writing or making a brushmark on the outside, it can always be told if anyone else tried to read it because the origami could never be folded to exactly match afterwards.

1: Start with a square
Write a note on the inner surface of the paper.

2: Fold the two sides in
This works best if you first fold and unfold the piece at each diagonal, to pre-crease it.